Apparatus and method for non-destructive testing of concrete

ABSTRACT

A device and method for determining characteristics of a concrete sample includes the use of multiple transducers. The transducers may couple to the concrete surface so that they can impart or receive mechanical waves from the sample. An area within a plane in the concrete sample is bounded by a collective extent of mechanical waves that pass through the plane and has a dimension at least as long as a distance between reinforcing bars in the concrete.

The present application is a continuation of U.S. application Ser. No.14/698,603, filed Apr. 28, 2015, now U.S. Pat. No. 9,939,420, whichclaims priority to U.S. provisional patent application Ser. No.61/986,029, filed Apr. 29, 2014, entitled APPARATUS AND METHOD FORNON-DESTRUCTIVE TESTING OF CONCRETE, the entire disclosure of each ofwhich is hereby incorporated by reference herein.

This invention was made with government support under contract numberW911NF-14-C-0010 awarded by the Department of Defense. The governmenthas certain rights in the invention.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by any-one of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent file or records, but otherwise reserves allcopyright whatsoever.

BACKGROUND

The present invention relates to the use of mechanical waves in thenon-destructive testing of concrete.

Methods and devices are known that utilize the propagation and receptionof mechanical waves within the acoustic and ultrasonic frequency rangesfor testing characteristics of concrete.

Ultrasonic pulse velocity (UPV) test methods utilize piezoelectrictransducers on opposite or adjacent sides of a concrete sample todetermine the velocity of an ultrasonic signal transmitted through theconcrete from one transducer to the other. Because defects in theconcrete, such as voids or delaminations, can affect ultrasonicmechanical wave speed through the sample, the amount of variation in thesignal velocity as measurements are taken across a concrete sample canindicate the presence of such defects or the presence of material suchas metal reinforcing bars (rebar). Further, the UPV test method can beused to determine compressive strength. As should be understood,compressive strength is an estimate of the maximum amount of force thatcan be applied normally to a surface of the concrete sample withoutcrushing the concrete.

ASTM C 597 describes a standard test method for utilizing pulse velocitythrough concrete. In one example of such method, respective transducersare disposed on opposite or adjacent sides of a concrete sample, such asa wall. Each transducer includes a piezoelectric element, as should beunderstood in this art, but other transducer crystals can be used. Animpedance matching material, which is used to decrease the impedancedifference between the piezoelectric material and the concrete, isdisposed between each transducer's piezoelectric element and theconcrete surface, and a gel is disposed between the impedance matchingmaterial and the concrete to fill air gaps. A control system excites oneof the two narrowband transducers to impart a pulse of ultrasoniclongitudinal mechanical waves (primary waves, or “p-waves”) into theconcrete surface, at a frequency ranging from 50 kHz to 120 kHz. Thepulse travels through the concrete and undergoes multiple reflections atoccurrences of density variations within the concrete, for example dueto delamination, air pockets, or rebar. A complex system of mechanicalwaves develops, including both p-waves and shear (or “s”) waves, andpropagates through the concrete. P-waves travel faster than s-waves, andwhere the transducers are disposed are opposite sides of the concretesample, the p-wave therefore first reaches the piezoelectric receivingtransducer, which in turn converts the p-wave into an electrical signal.The transit time (T_(p)) for the pulse to travel the known path length(L) is measured by the control system, and the longitudinal pulsevelocity (C_(p)) is given by the following equation:

$C_{p} = \frac{L}{T_{p}}$

The accuracy of the velocity measured by this method is a function ofthe accuracy of the measured distance (L) between the transducers andthe measured transit time (T_(p)). For the pulse velocity operationalmode, the programmable data acquisition has a sampling period (h), ortransit time resolution, e.g. of 0.1 microseconds using a 10 MHz clock.

Shear waves also reach the opposite side of the concrete wall, and usinga pair of shear wave transducers similarly disposed on opposite oradjacent sides of the concrete as the p-wave transducers, the s-wavetransit time (T_(s)) is similarly measured. As should be understood inthis art, s-wave transducers are piezoelectric devices configured tomechanically react in response to shear waves, thereby producing anelectrical signal when the transducer is affected by a shear wave. Giventhe path length (L), the shear velocity (C_(s)) is given by thefollowing equation:

$C_{s} = \frac{L}{T_{s}}$

The p-wave velocity (C_(p)) and shear velocity (C_(s)) are correlated tothe Young's modulus (E), Poisson's ratio (ν), and density (ρ) of thematerial as determined by the following equations:

$C_{p} = \sqrt{\frac{E\left( {1 - v} \right)}{{\rho \left( {1 - {2v}} \right)}\left( {1 + v} \right)}}$$C_{s} = \sqrt{\frac{E}{2{\rho \left( {1 + v} \right)}}}$

The p-wave modulus (M) is correlated to the p-wave velocity (C_(p)) anddensity (ρ) of the material as determined by the following equation:

M=ρC _(p) ²

Using shear wave transducers, the shear modulus (G) can be correlated toshear velocity (C_(s)) and density (ρ) of the material as determined bythe following equations:

G = ρ C_(s)²$v = {\frac{M - {2G}}{{2M} - {2G}} = \frac{C_{p}^{2} - {2C_{s}^{2}}}{2\left( {C_{p}^{2} - C_{s}^{2}} \right)}}$

Thus, the Poisson's ratio (ν) can be determined without knowing theconcrete density by measuring the p-wave velocity (C_(p)) and shearvelocity (C_(s)). Once the Poisson's ratio (ν) is known, the controlsystem calculates the Young's modulus (E) from the above equation, wheredensity (ρ) is known from empirical destructive (stress/strain) testing.For conventional concrete from 200 psi to 3,000 psi, the compressivestrength (σ) can be calculated from the following equation:

E=0.043ρ^(1.5)√{square root over (σ)}

Where density (ρ) is in units of kg/m³, and Young's modulus (E) andcompressive strength (σ) are in MPa, the above equation is applicable totwenty-eight day compressive strength, and the following adaption of theAmerican Concrete Institute (ACI) equation can be used, with the valueof the proportionality constant (k) determined by curve fittingexperimental data:

E=k√{square root over (σ)}

The American Concrete Institute (ACI) Committee 318 recommends a modelto predict the modulus of elasticity for a wide range of concretecompressive strengths from 200 psi to 3,000 psi, although overestimatingthe modulus of elasticity for compressive strength over 6,000 psi [ACI318-11].

E=0.043ρ^(1.5)√{square root over (σ)}

-   -   where:    -   E=modulus of elasticity in MPa    -   ρ=density in kg/m³    -   σ=compressive strength in MPa

E=4.38ρ^(1.5)σ^(0.75)

-   -   where:    -   E=modulus of elasticity in psi (English)    -   ρ=density in pcf or lb/ft³ (English)    -   σ=compressive strength in psi (English)

The ACI Committee 363 recommends a model for higher strength concretesranging from 3,000 psi to 12,000 psi [ACI 363R-92].

E=3320√{square root over (σ)}+6900

-   -   where:    -   E=modulus of elasticity in MPa    -   ρ=density in kg/m³=2323 kg/m³    -   σ=compressive strength in MPa

E=(40000√{square root over (σ)}+1.0×10⁶)(ρ/145)^(1.5)

-   -   where:    -   E=modulus of elasticity in psi (English)    -   ρ=density in pcf or lb/ft³ (English)=145 lb./ft³    -   σ=compressive strength in psi (English)

The Architectural Institute of Japan (AIJ) recommends an equation topredict the modulus of elasticity for high-strength concretes rangingfrom 2,900 psi to 23,200 psi [Tomosawa, et al 1990]. The AIJ equationexpresses the modulus of elasticity (E) as a function of compressivestrength (σ), and density (ρ):

E=k1486σ^(1/3)ρ²

-   -   where:    -   E=modulus of elasticity in MPa    -   ρ=density in kg/m³    -   σ=compressive strength in MPa    -   k=k₁k₂    -   k₁=correction factor corresponding to coarse aggregates    -   k₂=correction factor corresponding to mineral admixtures

Compressive strength may also be determined by acoustic attenuation orrelative amplitude, which measures the attenuation of an acoustic waveby observing the ratio of the wave amplitudes. As ultrasonic waves passthrough materials, attenuation is caused by beam divergence (distanceeffect), absorption (heat dissipation), and scattering. Scattering isthe only form of attenuation affected by the characteristics of thematerials through which the waves pass, as well as the degree ofinhomogeneity and frequency of the transducer. Attenuation caused byscattering (α_(s)) is given by:

$\alpha_{s} \propto \left\{ \begin{matrix}{{{1/D}\mspace{14mu} {for}\mspace{14mu} {diffusion}\mspace{14mu} {range}\mspace{14mu} \lambda} \leq D} \\{{{Df}^{\; 2}\mspace{14mu} {for}\mspace{14mu} {stochastic}\mspace{14mu} {range}\mspace{14mu} \lambda} \approx D} \\{{D^{3}f^{4}\mspace{14mu} {for}\mspace{14mu} {Raleigh}\mspace{14mu} {range}\mspace{14mu} \lambda}\operatorname{>>}D}\end{matrix} \right.$

where f is the wave frequency, λ is the wavelength, and D is the averageinhomogeneity in concrete. D may also be the void or aggregate size. Forλ much greater than D, concrete strength is related exponentially withthe wave attenuation.

Porosity is the main factor influencing strength of a brittle materialsuch as concrete. Several models that relate strength to porosity exist,but the most common is the exponential model:

K=K ₀ e ^(−kp)

where K₀ is the strength at zero porosity, P is the fractional porosity,and k is a constant that depends on the system being studied.

Techniques for determining ultrasonic attenuation include placement ofreceiving and transmitting transducers on opposite or adjacent sides ofa concrete sample. Typically, the use of adjacent sides is not possiblebecause the amplitude of the pressure and the torsion waves aredifficult to determine. However, when thickness of the structure isknown, it may be possible to utilize an impulse reflected off of theopposing surface of the concrete sample, assuming a sufficiently highinput signal.

When porosity is not known, the relative amplitude (β) can be correlatedto the fractional porosity (P) for a specific condition, as shown by thefollowing relationship between strength (K) and relative amplitude (β):

K=e ^(5.2115−0.1444β)

The equation above is applicable to concrete with a moisture content of3-4%, an age of ninety days, made from crushed granite aggregate with amaximum size of twenty mm, cured by immersing in water for twenty-eightdays, and measured by the direct technique (receiving and transmittingtransducers on opposing sides of the concrete sample) at 150 mm beampath distance without reinforcement bars. The relative amplitudedecreases as the strength is increased. While the above equation is anexample, such a relationship between strength and relative amplitude canbe drawn from empirical testing.

When an impulse is transmitted through a material, the relativeamplitude (β) is given by:

$\beta = {20{\log \left( \frac{A_{ps}}{A_{p}} \right)}}$

where A_(ps) is the pressure wave amplitude after the arrival of thetorsional wave, and A_(p) is the pressure wave amplitude. Since therelative amplitude method sends an impulse through the concrete, itmight also be used to correlate the size, type, and stiffness of anyreinforcing fibers. This correlation is determined by sending impulsesat various frequencies and analyzing the frequency response.

In some instances, only one side of the concrete sample may beaccessible, such that thickness of a concrete sample is unknown. In suchcircumstances, or otherwise where it is desired to determine thicknessof a concrete sample, the impact-echo method of determining concretethickness may be used, as described in the ASTM C 1383 standard. Theimpact-echo test involves two modes of operation, both of which relyupon mechanical waves imparted to a concrete sample by an impact hammer.The impact hammer produces a mechanical impact on the concrete surface,generating multiple modes of vibration, including p-waves, s-waves andRayleigh waves. The impact hammer includes a steel ball head in which isdisposed a piezoelectric element that generates an electrical signalwhen the steel ball strikes the concrete sample. The impact hammeroutputs this signal to a computer system, allowing the computer systemto recognize that the test has begun and to therefore configure thesystem to receive the receiving transducer output.

The first part of the test determines p-wave speed, based on receptionof the hammer-imparted p-wave detected by a pair of broadbandtransducers disposed on the same concrete surface at which the hammerimparts the mechanical wave. Both transducers may include piezoelectricelements that are coupled to the concrete surface. The receivingtransducers are independently disposed on the concrete surface at afixed distance, e.g. about 300 mm, apart. Although disposed on theconcrete surface independently of each other, a spacer may be placedbetween them to fix the desired distance. The operator strikes a hammeron the concrete surface on the same line that includes the centers ofthe two receiving transducers, at a distance of 150+/−10 mm from theclosest transducer, with an impact duration of 30+/−10 microseconds.

When the p-wave reaches the two piezoelectric receiver transducers, thetransducers convert the mechanical energy to an electrical signal thatis output to a computer. Upon reception of the signals from thereceiving transducers, the computer determines the difference in timebetween the two signals, i.e. the p-wave's time of travel between thetwo receiving transducers, or (Δt). Since the distance (L) between thereceiving transducers is known, the computer calculates p-wave speed(C_(p)) by dividing distance by travel time. P-wave speed in concrete isthen converted to the apparent p-wave speed in a plate(C_(p, plate)=0.96 C_(p)).

The second part of the test determines the frequency of a standing wavegenerated by the hammer impact, i.e. the resonance frequency. Abroadband transducer is manually disposed on the concrete surface, andthe operator strikes the same concrete surface with the impact hammernear the transducer. The piezoelectric element at the impact hammer headoutputs a signal from the hammer to the computer that triggers thecomputer to watch for a response from the broadband receivingtransducer. The impact generates a p-wave that propagates into theconcrete plate and reflects from the opposite surface. The return wavereflects, in turn, from the initial impact surface, and so on, givingrise to a transient thickness resonance. The broadband transducerconverts the detected wave into an electrical signal that is output tothe computer, which captures the output as a time domain waveform. Thecomputer obtains a frequency domain signal through a windowing functionand execution of a Fast Fourier transform. A Hamming window may be usedto reduce ringing in the spectral values outside the windows. A samplingperiod may be two microseconds, using a 500 kHz clock and 1024 datapoints in the recorded waveform. The duration of the recorded waveformis 2048 microseconds, giving a spectral resolution of 488 Hz in thesignal spectrum. There are 512 frequency channels, and the maximumsample frequency is 250 kHz. The computer displays 1024 samples in thetime domain and 512 bins (250 kHz) in the frequency domain. Theresonance frequency (f) appears as a peak in this waveform, which thesoftware application identifies.

Thickness of the concrete plate is then given by the following equation:

$T = \frac{C_{p,{plate}}}{2f}$

The actual impact has a significant influence on the success of theimpact-echo test. The estimate of the maximum frequency in the frequencydomain excited is the inverse of the impact hammer's contact time at theconcrete surface. Thus, a shorter contact time results in a higher rangeof frequencies contained in the pulse imparted into the concrete by theimpact hammer, and the depth of the opposing surface (which may be theopposite surface of the concrete sample, or a defect or object locatedwithin the sample that creates an intermediate standing wave) which canbe detected decreases according to the equation above. Short durationimpacts are needed to detect opposing surfaces and defects that are nearto the surface upon which the test is performed. Sansalone and Streett,“Impact-Echo: Nondestructive Evaluation of Concrete and Masonry,”(1997), provide an estimate of the maximum frequency (f_(max)=291/D) fora steel ball bearing of diameter D, and it is known for an impact hammerto utilize interchangeable steel and stainless steel balls that vary indiameter. As steel ball diameter increases in the impact hammer, so doesmaximum detectable thickness.

Depending upon knowledge of the characteristics of the concrete sample,the concrete density may be known.

The ultrasonic pulse echo method may be used on one side of a concretesample to determine both thickness and concrete characteristics in thesample when only one side of the sample is available. In particular,this method may be used to detect internal features, such as thelocation and density of rebar. The principle is based on the measurementof the time interval between transmitting an ultrasonic impulse into thesample and receiving an echo. The transit time (T) of the pulse totraverse twice the path length to (L) is measured, and the longitudinalpulse velocity (C_(p)) is given by:

$C_{p} = \frac{2L}{T}$

Ultrasound is highly attenuated in concrete, and for increasingthicknesses, it may therefore be difficult to effectively obtain an echosignal. Thus, to overcome the effects of wave scattering, and thusattenuation, caused by aggregates and air pores, the frequency of theultrasound signal is typically low, and can be as low as 50 kHz.

To implement this method, two narrowband transducers are applied to thesame side of the concrete sample, at a predetermined distance apart fromeach other. The computer system excites one of the two transducers,causing the transducer to impart a mechanical signal into the sample.The computer system is in communication with both the transmitting andreceiving transducers, actuating the transmitting transducer andreceiving the electrical signal from the receiving transducer. Thesignal received from the receiving transducer will include datadescribing both a surface wave and reflections. To remove the surfacewave data, leaving the reflection data, the computer system applies asignal processing technique known as frequency-wave number filtering (FKfiltering). FK filtering uses the slope of the data to selectivelyremove values that lie along a particular line (two dimensionalfiltering).

In essence, the pulse-echo method determines the time of flight of themechanical pulse imparted into the concrete sample and reflected backfrom the opposing side of the sample or an intermediate object, such asrebar. By taking these measurements sequentially across a concretesample, the most common detected distance is typically from the opposingsample side. Accordingly, anomalies of shorter distances that appear inthe output data correspond to positions at which imbedded material mayoccur.

SUMMARY OF THE INVENTION

In a method of determining a characteristic of a concrete sample in anembodiment of the present invention, at least one broadband transduceris disposed in contact with a surface of a concrete sample. A firstmechanical wave is imparted in the concrete sample at a positionproximate to the broadband transducer so that a standing wave isestablished in the concrete sample and so that the standing wave isdetectable by the at least one broadband transducer. The standing waveis detected at the at least one broadband transducer, and at least onecorresponding output signal is generated. A resonant frequency of thestanding wave is determined from the at least one broadband transduceroutput signal. A plurality of narrowband transducers are disposed incontact with the concrete surface at predetermined distances from eachother. At least one of the narrowband transducers is actuated so thatthe at least one transducer imparts a second mechanical wave at thesurface of the concrete sample. The second mechanical wave is receivedby at least one other narrowband transducer, and at least one respectiveoutput signal is responsively generated. Based on the at least onenarrowband transducer output signal, a velocity of the second mechanicalwave is determined. Based on the velocity of the second mechanical waveand the resonant frequency, a depth of a characteristic of the concretesample is determined.

A device for determining characteristics of a concrete sample in anotherembodiment of the present invention includes an impact device forimparting a first mechanical wave to a concrete sample so that astanding wave is established in the concrete sample. The device includesat least one broadband transducer for detecting the standing wave andresponsively generating an output signal, and a frame. A plurality ofnarrowband transducers is secured by the frame at predetermineddistances with respect to each other and so that coupling surfaces ofthe narrowband transducers are generally coincident to a surface shapecorresponding to a surface of the concrete sample. A control device isin communication with the broadband transducer and the narrowbandtransducers, and is configured to receive at least one output signalfrom the at least one broadband transducer, and actuate at least one ofthe narrowband transducers to impart a second mechanical wave at thesurface of the concrete sample. The control device receives at least oneoutput signal from and generated by respective at least one other of thenarrowband transducers in response to reception of the second mechanicalwave. The control device determines from the at least one broadbandtransducer output signal a resonant frequency of the standing wave.Based on the at least one narrowband transducer output signal, thecontrol device determines a velocity of the second mechanical wave, and,based on the velocity of the second mechanical wave and the resonantfrequency, determines a depth of a characteristic of the concretesample.

In a further embodiment, a device for determining characteristics of aconcrete sample includes a frame and a plurality of shear wavetransducers secured by the frame at predetermined distances with respectto each other and so that coupling surfaces of the shear wavetransducers are generally coincident to a surface shape corresponding tothe surface of the concrete sample. A plurality of primary wavetransducers is secured by the frame at predetermined distances withrespect to each other and so that coupling surfaces of the primary wavetransducers are generally coincident with the surface shape. A controldevice is in communication with the shear wave transducers and theprimary wave transducers and is configured to actuate at least one ofthe shear wave transducers so that the at least one shear wavetransducer imparts a shear wave in the concrete sample, and to actuateat least one of the primary wave transducers so that the at least oneprimary wave transducer imparts a primary wave in the concrete sample.The control device receives at least one output signal from respectiveat least one other of the shear wave transducers in response toreception of the shear wave by the at least one other shear wavetransducer. The control device receives at least one output signal fromrespective at least one other of the primary wave transducers inresponse to reception of the primary wave by the at least one otherprimary wave transducer.

In an additional embodiment, a device for determining characteristics ofa concrete sample includes a frame and a plurality of first transducerssecured by the frame at predetermined positions with respect to eachother and so that coupling surfaces of the first transducers aregenerally coplanar with each other, and a control device incommunication with the first transducers and configured to actuate atleast one of the first transducers so that the at least one firsttransducer imparts a mechanical wave in the concrete sample. The controldevice receives at least one output signal from respective at least oneother of the first transducers in response to reception of themechanical wave by the at least one other first transducer. Based on thereceived at least one output signal, the control device determines adepth of a characteristic of the concrete sample. The first transducersare arranged in the frame so that an area that is within a planeparallel to the coupling surfaces and in the concrete sample, and thatis bounded within the plane by an extent of the mechanical waves thatpass through the plane and that are receivable by the first transducers,has a dimension parallel to the coupling surfaces of at least about twofeet.

In a still further embodiment, a device for determining characteristicsof a concrete sample includes a frame and a plurality of firsttransducers secured by the frame at predetermined positions with respectto each other and so that coupling surfaces of the first transducers aregenerally coplanar with each other. A control device is in communicationwith the first transducers and is configured to actuate at least one ofthe first transducers so that the at least one first transducer impartsa mechanical wave in the concrete sample, and to receive at least outputsignal from respective at least one other of the first transducers inresponse to reception of the mechanical wave by the at least one otherfirst transducer. Based on the received at least one output signal, thecontrol device determines a depth of a characteristic of the concretesample. The first transducers are arranged in the frame so that an areathat is within a plane parallel to the coupling surfaces and within theconcrete sample, and that is bounded within the plane by an extent ofthe mechanical waves that pass through the plane and that are receivableby the first transducers, has a dimension parallel to the couplingsurfaces at least as long as the distance between reinforcing bars inthe concrete sample.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendeddrawings, in which:

FIG. 1A is a perspective view of a device for determiningcharacteristics of a concrete sample according to an embodiment of thepresent invention;

FIG. 1B is a side view of the device illustrated in FIG. 1A;

FIG. 1C is a side view of the device illustrated in FIG. 1A;

FIG. 1D is a bottom view of the device as in FIG. 1A;

FIG. 2 is an exploded view of a device as in FIG. 1A;

FIG. 3 is a schematic illustration of a narrowband transducer used inthe device as shown in FIG. 1A;

FIG. 4 is a schematic block diagram of control circuitry utilized in thedevice as in FIG. 1A;

FIG. 5 is a flow chart illustrating steps performed by applicationsoftware located at the control system of the device illustrated in FIG.1A;

FIG. 6 is a table illustrating operating characteristics of the deviceas shown in FIG. 1A;

FIG. 7 is a schematic illustration of an impact hammer and broadbandtransducer for use with the device as in FIG. 1A;

FIGS. 8A and 8B are graphical illustrations of signals generated bytransducer of the device illustrated in FIG. 1A;

FIG. 9 graphical illustrates a signal generated by a transducer of thedevice illustrated in FIG. 1A; and

FIGS. 10A-10E are screen displays of a graphical user interface operatedby the device illustrated in FIG. 1A.

Repeat use of reference characters in the present specification anddrawings is intended to represent same or analogous features or elementsof embodiments of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made in detail to presently preferred embodimentsof the invention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe invention, not limitation of the invention. In fact, it will beapparent to those skilled in the art that modifications and variationscan be made in the present invention without departing from the scopeand spirit thereof. For instance, features illustrated or described aspart of one embodiment may be used on another embodiment to yield astill further embodiment. Thus, it is intended that the presentinvention covers such modifications and variations as come within thescope of the present disclosure.

Referring to FIGS. 1A-1E and 2, a hand held device 10 for determiningcharacteristics of a concrete sample 12, for example a wall, includes ahousing 14 that encloses a transducer array unit assembly 16 and acontrol device comprised of a data acquisition unit assembly 18 and acontroller unit 20. Housing 14 is made from a glass-filled NYLON 12, orother suitable thermoplastic or other material that, together with frame22, forms a watertight device housing. Transducer array unit assembly 16includes a frame 22 having a rim 24 that is generally rectangularlyshaped and that is adhesively attached to a polymer trim 26 thatreceives rim 24 within a circumferential groove defined by trim 26. Agasket (not shown) is disposed between rim 24 and an open rim 27 ofhousing 14 to provide a watertight seal between the housing and theplate. The trim encloses the acoustic coupling surfaces of thetransducers held by frame 22 when device 10 is pressed against aconcrete surface, as discussed below, and serves as an electricalinsulator between the electronics of device 10 and the concrete sample.Rim 24 of frame 22 attaches to rim 27 of housing 14 with a rubber gasket(not shown) between the rim and the housing to provide a water/dustseal.

Frame 22 includes a generally planar plate 28 that is bounded by rim 24and that is continuous except for twenty circular holes at whichcorresponding transducer sleeves 30, 32, and 34 are attached orintegrally formed. Each sleeve 30, 32, and 34 is cylindrical in shapeand open at both ends. The openings in plate 28 correspond to therespective sleeve diameters. The metal at the sleeve base may be roundedat corners 36.

Each transducer sleeve 30, 32, and 34 has an inner diameter sized sothat the sleeve receives a respectively sized transducer 38, 40, and 42.Each sleeve defines a pair of slots 44 that extend longitudinally(parallel to the sleeve axis) on opposite sides of sleeve's cylindricalbody and that open to the open end of the sleeve opposite plate 28. Eachtransducer 38, 40, and 42 has a pair of pins or ridges 45 disposed onopposite sides of the transducer body and having a width correspondingto the width of the slot 44 in the sleeve in which the transducer isreceived. Sleeves 30 include one slot 44, for one corresponding rib 45on transducers 38, and a larger slot to receive a BNC connector, but itshould be understood that the sleeves can include two slots 44 toreceive two transducer ribs 45. Each ridge extends sufficiently radiallyoutward from the transducer's center axis so that the transducer'sopposing pins or ridges 45 are slidingly received in the correspondingslots 44 as the transducer is received through the open sleeve top endand into the sleeve. The bottom edge of each pin or ridge 45 comes torest at the closed bottom end of its corresponding slot 44, therebydefining the transducer's lower limit of travel toward and through theopenings in plate 28.

Although sleeves 30, 32 and 34 are shown in FIG. 2 as open on their endsopposite plate 28, this is for purposes of clarity and explanation only,and in the assembly, each sleeve has a cap (see cap 29) that is securedto the open sleeve ends (see 31) to retain the transducers in therespective sleeves. As illustrated in FIG. 2, each transducer includesone or more BNC connectors 62 that deliver electrical signals to (in thecase of transmitting) or deliver electrical signals from (in the case ofreceiving) the transducer. Each BNC connector is connected to a wiredlead that extends (e.g. through the sleeve or cap) and connects to acorresponding BNC connector on board 64, thereby connecting thetransducer to the board 64 circuitry. At each sleeve, a spring (notshown) is disposed between the cap and the transducer and biases thetransducer away from the cap in the direction toward and through plate28. When device 10 is not pressed to a concrete surface, the force ofthese springs pushes the transducers so that their ribs 45 engage thebottom of their respective sleeve slots 44. In this condition, thetransducer coupling surfaces 46, 48 and 50 of respective transducers 38,40 and 42 are coplanar, within a plane that is defined below (withrespect to housing 14) the plane defined by the open end of trim 26. Asthe user presses device 10 to a concrete surface, therefore, thetransducer coupling surfaces engage the concrete surface before trim 26,and as the user applies pressure to housing 14 until trim 26 engages theconcrete surface, the concrete surface reaction force pushes thetransducers back against their respective sleeve springs, causing thetransducers to slide back into their sleeves in the slots 44, toward thesleeve caps. The resulting spring pressure facilitates the coupling ofthe transducer coupling surfaces to the concrete surface and causes apliable, malleable solid coupling material (described below) between thecoupling surfaces and the concrete surface to deform in a manner to fillair gaps between the transducer coupling surfaces and the concretesurface. It will be apparent, as well, that although the presentdiscussion assumes a planar concrete surface, the individual transducermovement allowed by the sleeve/spring arrangement allows device 10 to beused as well on non-planar surfaces, as the transducers can accommodatesome variation from a plane. The force applied by the individual springsmay vary, but in the illustrated example each spring applies a forcewithin a range of about one pound to about three pounds at the point atwhich the transducers have pushed back against the springs so that thetransducer coupling surfaces are coplanar with the open end of trim 26.Because, as discussed below, the output signals from p-wave transducers40 and s-wave transducers 38 are processed with automatic gain control,greater flexibility can be allowed in the force with which they contactthe concrete surface, and in this example their springs apply a forcenear one pound. As also described below, however, the output signalsfrom transducers 42 are not subject to automatic gain control, and inthis example their springs apply a force of near three pounds.

When device 10 is placed on a generally planar concrete surface so thattransducers 38, 40, and 42 operatively engage the concrete surface, allof coupling surfaces 46, 48, and 50 are disposed with respect to theconcrete surface so that the given transducer can transmit or receivemechanical waves into or from the concrete sample at its operativefrequency range, depending on its mode of operation in the system.Transducer coupling surfaces 46, 48, and 50 engage the concrete samplesurface through solid coupling materials 56, 58, and 60, each sizedcorrespondingly to the coupling surface of its transducer. Each couplingpad 56, 58, and 60 couples its corresponding transducer 38, 40, and 42to the concrete by filling any spaces between the transducer couplingsurfaces and the concrete surface, eliminating air gaps that couldotherwise interfere with the transmission or reception of mechanicalwaves to or from the concrete. Such material should be pliable andmalleable, so that it can conform into the air gaps, and may be made,for example, of suitable composites as should be understood. Preferably,the coupling material is thin relative to transducer wavelength tolessen an impedance effect. In alternative embodiments, the dry materialmay be replaced by gel or liquid coupling materials, as should beunderstood. The use of solid, gel, or liquid materials to coupletransducers to materials such as concrete, should be understood and istherefore not discussed in further detail herein.

Transducers 38 are narrowband shear wave transducers, for example, asmanufactured by CTS Valpey Corporation, of Elkhart, Ind., Model No.SS0.058. Transducers 40 are narrowband p-wave transducers, for example,such as manufactured by Proceq SA and Proceq USA, Inc., of Aliquippa,Pa., Model No. 32540130. The construction of narrowband transducers 42is discussed in more detail below. Each of transducers 38, 40, and 42has disposed about its outer circumference one or more elastic O-ringsthat engage the inner circumferential surfaces of their correspondingsleeves to assist in locating the position of the transducer and toprovide a sealing engagement.

As should be understood, a piezoelectric transducer's bandwidth may beconsidered that part of the transducer's frequency response that iswithin 3 db (or half power) of the peak response. The transducer's Q, orquality, factor is the peak frequency in the response, divided by thetransducer's bandwidth. A narrowband transducer is one that can beconsidered to operate at a single frequency, whereas a broadbandtransducer operates over a range of frequencies. The contrast between asingle frequency and a range of frequencies depends on the context ofthe device, and thus the definition of what constitutes narrowband andbroadband can vary as appropriate for the circumstances.

Transducer array unit assembly 16 also includes the printed circuitboard 64 at which is disposed circuitry that controls voltage pulseamplifier, multiplexer, signal reception and processing, and LEDfunctions, so that the transducers and control circuitry perform thefunctions described herein. Board 64 is received and secured in theinterior of housing 14 so that when transducers 38, 40, and 42 arereceived in their respective sleeves 30, 32, and 34, and frame 22 issecured to housing 14, each of the transducers makes electrical contactwith the circuitry of board 64 via BNC connectors and associated leads.

The transducer array includes three shear wave transducers 38 and threeprimary wave transducers 40. In each group of three transducers, thecontrol device operates one transducer as a transmitter and the othertwo as receivers. The centers of the circular coupling surfaces of eachp-wave and s-wave transducer can be considered the transducers'operative centers. Considering receiving shear wave transducers 38 a and38 b, a line 66 passes through their centers, and transmitting shearwave transducer 38 c is considered to be aligned linearly with itsreceiving transducers 38 a and 38 b in that its operative center alsolies generally on line 66. Similarly, transducers 40 include a pair ofreceiving transducers 40 a and 40 b and a transmitting transducer 40 c.The operative centers of transducers 40 a and 40 b also define a linethat also generally passes through the operative center of transmittingtransducer 40 a. In this example, transducers 40 and transducers 38 are,additionally, collinear with each other so that all transducers 38 and40 are generally aligned on line 66, although in other embodiments, thetransducer groups are not collinear. Still further, the transducergroups 38 and 40 overlap in that transducers 40 are disposed betweentransmitting transducer 38 c and the nearest receiving transducer 38 b.Due to the collinear, spatially overlapped arrangement of thetransducers, the mechanical waves transmitted and received by the twotransducer groups at least partially share a common path through theconcrete, thereby increasing the similarity of conditions experienced bythe s-waves and the p-waves by reducing the impact of concrete'sinherent non-homogeneity. This beneficially impacts the reliability ofthe resulting measurements.

The coupling surfaces of transducers 42 are also circular incircumference. In the illustrated embodiment, there are fourteentransducers 42 secured by plate 28, and the plate holds the transducersso that their operative centers are aligned collinearly generally alonga line 68. Lines 68 and 66 are parallel to each other, although itshould be understood that other arrangements are possible.

In the embodiment described herein, the diameters of the couplingsurfaces of shear wave transducers 38, primary wave transducers 40, andprimary transducers 42 are about 1.25, two, and one inch, respectively.Preferably, the transducers are acoustically isolated from the plate,e.g. by the elastic O-rings so that the transducers do not inducevibrations in the plate capable of actuating other transducers held bythe plate. Considered along line 66, the operative centers of receivingp-wave transducers 40 a and 40 b are separated by a distance 70 of aboutthree inches. The operative center of transmitting p-wave transducer 40c is separated from the center of its closest receiving transducer 40 aby a distance 72 of about six inches. The centers of receiving shearwave transducers 38 a and 38 b are separated by a distance of 74 ofabout three inches, and the center of transmitting shear wave transducer38 c is separated from the center of its closest receiving transducer 38b by a distance 76 of about eighteen inches.

Considered along line 68, the centers of each pair of adjacenttransducers 42 are separated by a distance 77 of about 1.5 inches, andthe centers of the two furthest transducers 42 are separated by adistance 78 of about 19.5 inches. Generally, the separation between thetwo endmost transducers 42 is sufficient so that the measurementsconducted by the linear array can be expected to detect the presence ofrebar, even if the rebar is separated by its greatest expectedseparation, and regardless where the device is applied to the concretesample. Considered from the viewpoint of a plane parallel to thecoupling surfaces of transducers 42 within the concrete sample at adepth at which the rebar is expected, the linear dimension, parallel toline 68, of the intersection of the combined signal cones from and tothe transducers 42 and this plane is at least as long as the longestexpected separation of reinforcing bars.

As apparent from FIGS. 1A and 1D, the transmitting and receiving s-wavetransducers 38 are separated by a longer distance than are thetransmitting and receiving p-wave transducers. Because the p-wavecomponent of the signal imparted by the transmitting s-wave transducertravels faster than the s-wave component, the longer separation betweenthe shear wave transmitter and receivers allows greater separationbetween the wave components when they reach the receivers, decreasingthe likelihood that the p-wave component contributes significantly tothe detection of the s-wave component by the receiving s-wavetransducers.

As described in more detail below, transducers 42 are actuated insequential pairs, beginning at one end of the row along line 68.Consider, for example, the leftmost transducer 42 on line 68, in theperspective as in FIG. 1A (or, the bottommost transducer 42 in FIG. 1D).Upon actuation of a rebar location test described below, the controldevice excites this transducer to impart a mechanical pulse into theconcrete surface and selects the transducer to its immediate right fromwhich to receive an electrical signal corresponding to the resultingmechanical wave it detects. After a predetermined settling time, thecontrol circuitry then actuates the second transducer 42 on line 68(i.e., the immediately previously receiving transducer) to impart thenext mechanical wave into the concrete sample, and selects theimmediately adjacent transducer 42 to the right on line 68 from which toreceive the next data signal. This cycle continues, with the operativepair of transducers 42 sequentially shifting one transducer to the rightfor each measurement, until the rightmost transducer 42 (or, topmosttransducer, in FIG. 1D) functions as the receiving transducer. Thecontrol device then actuates the last (rightmost) transducer as thetransmitting transducer, with the transducer immediately to the left asthe receiving transducer. Accordingly, in one complete cycle of thefourteen transducers, all the transducers function as a transmitter, andall but the leftmost transducer function as a receiver (with the next tolast right transducer operating as a receiver twice), and there arefourteen operative pairs of transducers within the group of fourteentransducers 42.

As noted, in the presently described example, the diameter oftransducers 42 (including the transducer housing, although for purposesof explanation, this can also be considered the diameter of thetransducer's coupling surface) is about one inch. This diameter is inturn related to the concrete characteristic that device 10 is configuredto detect, in this instance steel reinforcing bars (rebar), and inparticular to the resolution needed to detect the rebar. As should beunderstood, reinforcing bars in large, generally planar-type concreteelements, e.g. walls and slabs, are commonly spaced about six inchesapart. Generally, however, rebar spacing can vary in typical concretestructures from about three inches to about eighteen inches. Because theminimum rebar spacing is expected to be about three inches, the array oftransducers 42 should operate with a resolution of at least three inchesif it is desired to maintain the capability to distinguish betweenadjacent bars. To accurately identify the rebar occurrences according tothe Nyquist criterion without spatial aliasing, then, the maximumspacing between sampling points is half the desired resolution, or 1.5inches. If it is assumed that each transmitting/receiving pair oftransducers 42 detects the presence of rebar at points on a linebisecting the parallel axes of the two adjacent transducers, then thesebisecting lines should be spaced apart by a maximum distance of 1.5inches, and since each operative pair of transducers 42 is shifted onetransducer spacing from the previous pair, this maximum distance is alsothe maximum distance between the centers of adjacent transducers 42.Because the distance between transducers corresponds to the distancebetween the transducers' centers, a maximum transducer spacing of 1.5inches means that the transducers should be less than 1.5 inches indiameter, and to allow suitable isolation, preferably about one inch orless.

It should be understood, however, that larger-diameter transducers maybe used within the scope of the present disclosure. In that event,unless the actual rebar spacing were known, and unless that spacing werewithin the device's resolution, the device would be able to identify thepresence of rebar, but not confidently identify a given bar's positionwith respect to an adjacent bar. Accordingly, in such embodiments, theidentifying array 108 of LEDs (described below) might be omitted infavor of a single LED or commonly activated LED array that is activatedwhenever the device identifies the presence of rebar.

Returning to the present example, because the maximum expected spacingof rebar is eighteen inches, and given the transducer spacing of about1.5 inches, fourteen transducers are used in order to assure that whenthe device is pressed onto a concrete surface in two 90° offsetpositions, the linear array will detect rebar if it is present in thesample. This results in an array of transducers 42 of about 19.5 inches,with a resulting length of housing 14, in the dimension of lines 66 and68, of about two feet. As should be appreciated by the presentdisclosure, however, the number, dimensions, and arrangement oftransducers 42 may vary as desired, and in particular with respect tothe concrete characteristic being measured. The device as presentlydescribed in these examples weighs about ten pounds, and in this examplethe about ten pound weight and about two feet maximum housing dimensionallow the device to be used as a handheld device. It should beunderstood, however, that these dimensions and weight can vary, and forexample the present invention contemplates the device 10 constructed ata weight less than about ten pounds and/or with a major dimension in aplane parallel to the plane of plate 28 shorter than about two feet.

Similarly, in other embodiments, parallel rows of transducers 42 may beutilized, i.e., a two-dimensional or other multi-dimensional array.Multi-dimensional arrays may be desirable, depending on the measured-forcharacteristic. In this presently-described example of an array utilizedfor detecting rebar, however, a linear array is effective and alsobeneficially reduces the size of device 10. Because rebar is typicallydisposed in concrete as parallel bars, an operator may make twomeasurements at a 90 degree offset with respect to each other, butotherwise at the same or similar location on the concrete surface, andhave confidence that if the rebar is uniformly distributed within theconcrete sample, one or both of these two measurements should detect itspresence.

Referring to FIG. 3, each transducer 42 has a generally cylindricalhousing 82 made of polyether ether ketone, or PEEK, that defines agenerally central groove 84 in which is disposed an elastic O-ring 86that engages the inner circumference of the transducer sleeve 34 (FIG.2) in which the transducer is received. Housing 82 defines a generallycylindrical inner bore 88 in which is disposed a piezoelectric element90 formed of a piezoelectric epoxy composite material, for example apiezoelectric ceramic known as PZT-5H2, that has a thickness ofapproximately one inch, an acoustic impedance of 12.1 MRayls, and aresonance frequency of 56 kHz. A backing material 92 fills a chamberimmediately behind piezoelectric element 90. Backing 92 is a dielectricmaterial that holds the piezoelectric element 90 in place and dampensringing in the piezoelectric element after the initial pulse. In oneembodiment, backing 92 is comprised of particles of various sizes oftungsten mixed with an epoxy resin in a ratio such that the compositehas a resultant acoustic impedance of 12 MRayls. A front face 94 isdisposed at the forward end of piezoelectric element 90 and defines thecoupling surface 96. Front face 94, in this example, is made of a glassceramic, for example sold under the name MACOR available from Corning,Inc., of Corning, N.Y., that has an acoustic impedance of 11.7 MRayls.Front face 94 acts as a buffer plate that protects piezoelectric element90 from the rough concrete surface and that is stable at hightemperatures without significant thermal expansion. The front facereduces the impedance mismatch between the piezoelectric element and theconcrete surface. A thin dry solid couplant layer 98 (identified as 60in FIG. 1A) couples the transducer to the concrete by filling any gapsbetween front face 94 and the concrete surface, eliminating air gapsthat could otherwise interfere with the transmission of mechanical wavesto the concrete from the transducer.

As should be apparent from the present disclosure, the transducersdescribed herein may operate in the acoustic or ultrasonic frequencyranges, but for purposes of explanation may be referred to generally asacoustic transducers having acoustic coupling surfaces.

Data Acquisition Unit 18 includes a printed circuit board 100 thathouses a main microprocessor that controls the operation of device 10,as well as a dedicated microprocessor for automatic gain control. Theorganization and operation of the circuitry of board 100 is discussed inmore detail below. Batteries and a sealed battery compartment (notshown) may be provided in housing 14 as a power source, and/or DC powermay be supplied by an external power source via a power input port inhousing 14.

The control device also includes a controller unit 20, in this example amobile computing device that comprises one or more microprocessors thatexecute an operating system that supports programmable computingfeatures and the installation of application programs. Mobile device 20includes input/output capability that allows operative connection to thecontrol circuitry at board 100. Device 20 also includes a display driventhrough the operating system by a graphical user interface, as describedbelow. In one embodiment, controller unit 20 is a commercially-available“smart phone” that is received in a correspondingly shaped cavity 102 inhousing 14. A cover plate (not shown) may be provided at housing 14 tohold controller unit 20 in place. It should be understood thatcontroller unit 20 can comprise another type of computing or mobiledevice, such as a tablet, or may comprise a dedicated processor andrelated circuitry disposed on a circuit board secured within acorresponding cavity 102.

A LEMO connector 104 allows communication between the microprocessor atboard 100 and an impact hammer/broadband transducer assembly 176 (FIG.7), as described below. A pair of test initiation buttons 107 (FIG. 1C)are also mounted in housing 14 near handles 52 and 54 (for ease ofaccess by the user holding the device) and are connected to themicroprocessor at board 100 through a general purpose input/output toestablish an actuation signal, as described below. A master switch 109selectively switches a USB connector (located between an on/off switch106 and LEMO connector 104) between operation as a power jack and as ameans for connecting an alternate computer device. On/off switch 106activates and deactivates device 10 and its power source. A fuse isprovided in a holder 113, and a DC power input jack is provided at 111.Fourteen LEDs of linear array 108 are mounted in housing 14 and aredriven by an LED driver board portion of board 64 and controlled by themicroprocessor at board 100. As described below, those LEDs correspondto transducers 42, and in the illustrated example, the LED array isaligned proximate the bottom edge of housing 14, and the LEDsapproximately spaced, so that the LEDs are spatially aligned withrespective transducers 42.

Referring to FIG. 7, a transducer/impact hammer assembly 176 includes animpact hammer 178 and a broadband transducer 180. Impact hammer 178includes a hand gripable handle 182 from which extends a semi-rigidhollow polymer tubing 184, at the end of which is interchangeablydisposed a one-and-three-eighths inch (or other size, as selected by theuser) steel ball 186. A piezoelectric element is disposed within steelball 186 and is connected to an output wire (not shown) at a sheathing188. The output wire extends through handle 182 to a harness 190 atwhich the wire is insulated from, but mechanically joined to, aninput/output wire to broadband transducer 180, in an overall devicecommunication wire 192. Wire 192 has a communication jack (not shown)disposed on its opposing end, for insertion in LEMO connector 104 (FIG.2).

As discussed herein, the actual impact applied by hammer ball 186 has asignificant influence on a success of the test, and impact hammer 178utilizes interchangeable steel balls that vary in diameter accordingly.As steel ball diameter increases, the detectable resonant frequency of asignal imparted by the hammer decreases, and the maximum detectablesample thickness thereby increases. The one-and-three-eighths inch ballis effective at least for concrete walls of about two, three and sixfeet in thickness, although it should be understood this may vary.Because the direction of the mechanical wave imparted to the concretevaries directly with the size of the steel ball, and because theimpact's duration should be shorter than the time needed for theresulting wave to traverse the sample's thickness, a smaller steel ballmay be used where concrete thickness is expected to be low.

Transducer 180 includes a steel housing 194 that encloses an internaltransducer assembly constructed generally similarly to the transducerdescribed with respect to FIG. 3, with a spring biasing the internaltransducer assembly so that the internal transducer assembly's frontface 196 extends forward at the distal portion of transducer 180. Thebroadband transducer comprises a piezoelectric crystal mounted on atungsten epoxy backing containing tungsten particles of 250, 25, andless than 1 micron in diameter. Electrical connections pass through thebacking to the crystal. A MACOR facer is disposed in front of thecrystal and defines the transducer's coupling surface. The piezoelectriccrystal is preferably composed of an epoxy/PZT composite having anacoustic impedance of 12 MRayls with an operation between zero Hz and 50KHz, but the ratio of PZT to epoxy can be adjusted to match theworkpiece material. As described above, a solid, dry coupling materialis secured to the acoustic coupling surface of the piezoelectrictransducer element in order to couple the transducer element to theconcrete surface. The dry contact pad has a similar response curve tothe ultrasonic gel that is commonly used in acoustic testing.

FIG. 4 provides a schematic functional diagram of the control device andcertain devices with which it interacts, and FIG. 5 illustrates theoperation of device 10 from the perspective of application softwareoperating on controller unit 20. A microprocessor 110 generally controlsthe operation of the concrete characteristic tests described herein (ascontrolled in turn by the application software), the provision of datato controller unit 20, and the operation of LED array 108. When the useractuates a start button (not shown) provided by a graphical userinterface drive by application software that operates on controller unit20 and that provides a screen display through which the user respondsvia a touch screen, the application begins operation, at 144. At 146,the application software may, in one embodiment, query the user (via adisplay 140 (FIG. 2) and graphical user interface at controller unit 20)to enter an expected rebar depth. Upon receiving a response, or inanother embodiment, without a rebar query, directly upon receiving theuser start button instruction, the application software causes thegraphical user interface to display (at 194, FIG. 10A) instructions tothe user to press device 10 (FIG. 1A) to the desired surface of theconcrete sample 12, so that the coupling surfaces of transducers 38, 40,and 42 (FIGS. 1A-1E) engage the concrete surface, and to press one ofthe two test initiation buttons 107 proximate handles 52 and 54. Theapplication program then enters a waiting mode, waiting for receipt of asignal from microprocessor 110 that a button 107 has been actuated anddetected by the microprocessor over a general purpose I/O 112. Whencontroller unit 20 receives this signal, the application software causesthe controller unit to send a signal back to microprocessor 110,instructing the microprocessor to initiate a p-wave velocity test. Inresponse, at 148, microprocessor 110 adjusts the output voltage level ofa voltage pulse amplifier module 124 via general purpose I/O 126 and acontrol module 125. In the presently described example, the pulseamplifier module is variable, so that microprocessor 110 controls theamplifier module to produce an actuating pulse over a range ofamplitudes up to about 1000V and at a desired pulse width. The length ofthe pulse is programmable via the application software between 2 and 20microseconds. Thus, the signal from microprocessor 110 includesparameters sufficient to establish the amplitude, frequency, and timeduration of a pulse initiated by the amplifier module. Control module125 comprises circuitry that translates the microprocessor instructionsinto suitable signals to drive the amplifier module. The configurationand function of circuitry such as control module 125, as well as controlmodules 130, 136, and 144, discussed below, should be well understoodand are therefore not discussed in further detail.

In the presently described example, the resonant frequency of p-wavetransducers 40 is about 54 kHz. As should be understood, mechanicalwaves attenuate in concrete, and a relatively low frequency is thereforepreferred in this example for the p-wave and s-wave transducers,although it should be understood that the frequency can vary as desired,provided the response signal can be acquired. Accordingly, theinstruction from microprocessor 110 is to generate a pulse at 54 KHz,although the pulse frequency is selectable between about 50 kHz andabout 120 kHz. Further, as discussed below, the presently illustratedp-wave velocity test is based on a p-wave time-of-flight fromtransmitting transducer 40 c to each of the receiving transducers 40 aand 40 b. Provided the receiving transducers' output signals aresufficiently strong that the received p-wave can be detected over noise,then, the velocity test is based on identification of time differentialrather than the ability to distinguish a particular amplitude level.That is, information is carried by a signal's time component rather thanits amplitude component. This introduces the possibility of errorarising from rise times. Because amplitude is not an informationcarrier, however, and because rise times can be improved by increasingsignal amplitude, microprocessor 110 in the present example instructsthe amplifier module to generate a pulse of about 592V (althoughamplitude can be varied as desired up to about 1000 v, which at thespacing of about six inches between transmitting transducer 40 c andfirst receiving transducer 40 a and of about nine inches betweentransducer 40 c and second receiving transducer 40 b, drives thereceiving transducers into saturation upon receipt of the resultingp-wave.

Simultaneously, microprocessor 110 actuates a 1:20 switch matrix 128(via a control module 130) to direct the output of amplifier module 124to p-wave transducer 40 c. Switch matrix 128 is a switching modulecomprised of discrete electronic components that can selectively directthe output of amplifier module 124 to any of transducers 38 c, 40 c andthe fourteen transducers 42, as discussed above.

When voltage pulse amplifier module 124 is ready to transmit, it sends apulse to microprocessor 110 that, in turn, notes the time and sends aninstruction signal back to module 124, causing module 124 to drivetransducer 40 c that, in turn, imparts a mechanical wave into theconcrete sample, resulting in a dominant p-wave that travels totransducer 40 a.

Also via bus 132, microprocessor 110 actuates a 20:1 switch matrix 134via a control module 136 to direct the output from receiving transducer40 a to an input selector relay 116. Relay 116 is a small solenoid relaythat acts as a two-way switch, though as should be understood, anelectrical switch could be used. Microprocessor 110 controls relay 116to move to one of its two states, at which the relay directs the outputof switch matrix 134 to an amplifier module 118. Amplifier module 118outputs the transducer output to an analog-to-digital converter 120 forstorage at a static random access memory 122 and retrieval bymicroprocessor 110. Thus, the receiving transducer 40 a detects theresulting wave and outputs a corresponding analog signal to amplifiermodule 118 via switch 134 and relay 116. Amplifier module 118 applies anautomatic gain control function to bring the signal strength to adesired level for acquisition by the system components. The amplifiedanalog signal is converted from analog to digital by converter 120 andsaved in SRAM 122. Microprocessor 110 controls the sampling load and thenumber of samples taken by converter 120.

Microprocessor 110 acquires the data from SRAM 122 in real time, andthus can analyze the retrieved data relative to the time themicroprocessor received the pulse from amplifier module 124 at actuationof the p-wave. In one embodiment, the microprocessor directs theretrieved data, and the time at which the transmitting transducer wasactuated, up to controller unit 20, where the application softwareanalyzes the retrieved data and determines the point at which therepresented signal (FIG. 8A) exceeds a predetermined thresholdindicating reception of the p-wave at the receiving transducer 40 a. Thethreshold can be determined through testing and calibration. Uponlocating the p-wave reception, the controller unit determines the timedifference (i.e. the time of flight) between the actuation of thetransmitter transducer to impart the p-wave into the concrete and thereception of the wave by the receiving transducer. In anotherembodiment, the microprocessor conducts the analysis and providesresults to the controller unit.

After a sufficient time for settlement, the microprocessor againinstructs voltage pulse amplifier 124 to ready a p-wave pulse fortransducer 40 c but now instructs 20:1 switch matrix 134 to direct theoutput of second receiving transducer 40 b to relay 116 and, therefore,to amplifier 118. Through an otherwise identical process, microprocessor110 or the controller unit determines the time period between initiationand reception of the resulting p-wave between transducers 40 c and 40 b(see FIG. 8B).

The application software (either directly from its analysis or uponreceiving time of flight results from microprocessor 110) determines adifference between the two times of flight, which is in turn due to thedifference in propagation time of the p-waves between the twotransducers 40 a and 40 b, as illustrated by comparison of the signalrepresentations provided by FIGS. 8A and 8B. Because the applicationsoftware also knows the distance between transducers 40 a and 40 b, theapplication software determines the p-wave velocity by dividing theknown distance by the differential time period.

At 150, the application software causes controller unit 20 to send asignal to microprocessor 110, causing the microprocessor to initiate thes-wave velocity test. Microprocessor 110 adjusts the output voltagelevel of voltage pulse amplifier module 124 via control module 125 andselects s-wave transducer 38 c to connect to the output of amplifiermodule 124 via control of switch matrix 128 by control module 130.Microprocessor 110 also selects, through switch matrix 134 and controlmodule 136, the output of transducer 38 a to output to amplifier module118 via actuation of input selector relay 116. When voltage pulseamplifier module 124 is ready to transmit, it sends a pulse tomicroprocessor 110 that, in turn, sends an instruction signal back tomodule 124, causing module 124 to drive transducer 38 c that, in turn,imparts a mechanical wave into the concrete sample, resulting in ans-wave that travels to transducer 38 a. The receiving transducer detectsthe resulting wave and outputs a corresponding analog signal toamplifier module 118 via switch 134 and relay 116. Amplifier module 118applies an automatic gain control function to bring the signal strengthto a desired level for acquisition by the system components. Theamplified analog signal is converted from analog to digital by converter120 and saved in SRAM 122. Microprocessor 110 controls the sampling loadand the number of samples taken by converter 120.

As should be understood, the transducers described herein impart bothp-waves and s-waves into the concrete. Although transducer 38 c is ans-wave transducer, and although the resulting s-wave imparted to theconcrete by transducer 38 c is the dominant wave, i.e. is much larger inamplitude than the imparted p-wave, the p-wave travels faster throughthe concrete than does the s-wave, with the result that the leading edgeof the p-wave will reach the receiving s-wave transducer before theleading edge of the s-wave. Thus, although the s-wave amplitude is muchhigher than the p-wave amplitude, to the extent the p-wave amplitude ishigher than the threshold amplitude, controller unit 20 (ormicroprocessor 110) could interpret the first-received p-wave to be thearrival of the s-wave without programming to discriminate between thetwo (such programming being employed in another embodiment).Accordingly, in the present example, microprocessor 110 instructsvoltage pulse amplifier 124 to prepare a pulse of an amplitudesufficiently low that the resulting s-wave reaching the receivingtransducer will cause the receiving transducer to output a signal thatwill exceed the threshold applied by the microprocessor for recognizingreceipt of an s-wave but so that the signal from the receivingtransducer corresponding to the first-received p-wave will not.Accordingly, in this example, the microprocessor instructs the voltagepulse amplifier module to prepare a 42 v (peak to peak) at 50 KHz (thes-wave transducers' resonant frequency) at a width of about 8.4microseconds.

Microprocessor 110, having acquired the sample data, directs the data tothe controller unit, at which the application software (or themicroprocessor) determines the s-wave time of flight in the same manneras it determined the p-wave time of flight. The application softwareknows the time at which the microprocessor instructed actuation of thetransmitting transducer and the time at which the microprocessoracquired the output signal from the respective receiving transducer. Thedifference between these times is the time of flight.

At completion of this data acquisition, and after a settling period,microprocessor 110 repeats the sequence with the same transmittingtransducer 40 c, but now utilizing transducer 40 b as the receivingtransducer. The microprocessor instructs the amplifier module to preparea second s-wave pulse, instructs switch matrix 134 to direct the outputof second receiving s-wave transducer 138 b to relay 116 and amplifier118, triggers application of the input pulse to the transmittingtransducer, and directs the resulting data to controller unit 20 so thatthe application software determines a time of flight of the resultings-wave between transmitting transducer 38 c and second receiving s-wavetransducer 38 b.

The application determines a difference between the two times of flight,which is in turn due to the difference in propagation time of thes-waves between the two transducers 38 a and 38 b. Because theapplication software also knows the distance between transducers 38 aand 38 b, the application software determines the s-wave velocity bydividing the known distance by the differential time period.

Given the now-determined p-wave velocity (CP) and s-wave velocity (CS),and assuming a concrete density of 2.38 g/m3, at 152 the applicationsoftware relates these three variables to concrete compressive strength,based on the following model provided by the Architectural Institute ofJapan (AIJ):

E=k1486σ^(1/3)ρ²

The p-wave velocity (C_(p)) and shear wave velocity (C_(s)) arecorrelated to the Young's modulus (E), Poisson's ratio (ν), and density(ρ) of the material as determined by the following equations:

$C_{p} = \sqrt{\frac{E\left( {1 - v} \right)}{{\rho \left( {1 - {2v}} \right)}\left( {1 + v} \right)}}$$C_{s} = \sqrt{\frac{E}{2{\rho \left( {1 + v} \right)}}}$

The p-wave modulus (M) is correlated to p-wave velocity (C_(p)) anddensity (ρ) of the material as determined by the following equation:

M=ρC _(p) ²

The shear modulus (G) can be correlated to shear velocity (C_(s)) anddensity (ρ) of the material as determined by the following equation:

G=ρC _(s) ²

The Poisson's ratio (ν) is correlated to the p-wave modulus (M) andshear modulus (G) as determined by the following equation:

$v = {\frac{M - {2G}}{{2M} - {2G}} = \frac{C_{p}^{2} - {2C_{s}^{2}}}{2\left( {C_{p}^{2} - C_{s}^{2}} \right)}}$

The Young's modulus (E), also identified as the modulus of elasticity,is correlated to shear modulus (G) or p-wave modulus (M) and Poisson'sratio (ν) as determined by the following equation:

$E = {{2{G\left( {1 + v} \right)}} = \frac{{M\left( {1 + v} \right)}\left( {1 - {2v}} \right)}{\left( {1 - v} \right)}}$

As noted above, modulus of elasticity of concrete is frequentlyexpressed in terms of compressive strength, but actual correlationbetween the two involves a model that can account for variations inconcrete properties and binder/aggregate proportions. These models, inturn, tend to be applicable to ranges of concrete strengths. TheAmerican Concrete Institute (ACI) Committee 318 recommends, for example,a model to predict the modulus of elasticity for a wide range ofconcrete compressive strengths, from 200 psi to 3,000 psi, but thatoverestimates the modulus of elasticity for compressive strength over6000 psi. The ACI Committee 363 recommends a model for higher strengthconcretes ranging from 3000 psi to 12,000 psi. The ArchitecturalInstitute of Japan (AIJ) recommends the equation above to predictmodulus of elasticity for high-strength concretes ranging from 2,900 psito 23,200 psi, and it is believed that the AIJ test successfullypredicts the modulus of elasticity up to about 28,000 psi. Although itshould be understood that various models may be employed within thescope of the present disclosure, in the present example the applicationsoftware utilizes the AIJ model, for prediction of modulus of elasticityfor compressive strength up to about 28,000 psi. That is, upondetermining the p-wave velocity (C_(p)) and s-wave velocity (C_(s)) fora given test, and assuming a density for concrete, the applicationsoftware correlates those determined and assumed values to Poisson'sratio (ν) and Young's modulus (E). Then, and again assuming a concretedensity, the AIJ model above relates the determined Young's modulus (E)to compressive strength (σ), where k=k₁*k₂, k₁ is a correction factorcorresponding to coarse aggregates, and k₂ is a correction factorcorresponding to mineral admixtures. In the presently describedembodiment, concrete density is assumed to be 2.38 g/m³, and k isassumed to be one. In another embodiment, however, the applicationsoftware queries the user to enter an actual density prior to the test'sbeginning, and if the user enters a value (through the graphical userinterface and screen 140), the application software determinescompressive strength based on the entered density value. Otherwise, theapplication uses the 2.38 g/m³ by default. Further, it should also beunderstood that the default values for density and k may change asdesired, for example as determined through destructive compressivestrength testing of concrete samples of known density andaggregate/admixture characteristics.

Upon determination of the compressive strength at 152, the applicationsoftware stores the result and causes controller unit 20 to send asignal to microprocessor 110 to initiate the rebar test, at 154. Inresponse, microprocessor 110 sends a command to control module 130 toset up the first (leftmost in FIG. 1) narrowband transducer 42 of thelinear array to receive a driving pulse from voltage amplifier 124through 1:20 switch matrix 128. Through control module 125,microprocessor 110 correspondingly instructs voltage pulse amplifiermodule 124 to prepare an 850 v_(pp), 8.4 microsecond pulse. Because, asnoted herein, the system acquires this test data without application ofautomatic gain control, the system may adjust the pulse amplitude toavoid saturation of the receiving transducer. When amplifier module 124is ready to transmit, it sends a pulse back to microprocessor 110, andupon receiving an instruction from the microprocessor, provides a highvoltage pulse output to the selected transmitting transducer 42. Thiscauses the transmitting transducer to impart a mechanical wave into theconcrete sample, causing a resulting p-wave to travel into the concreteand reflect from the opposing side of the concrete or from anintermediate structure or concrete defect. Again, a resonant frequencyin the range of 50 KHz is chosen in this example to reduce attenuationeffects in concrete, but it should be understood that other frequenciescan be utilized, e.g. up to about 120 kHz. Microprocessor 110 sends asignal to control module 136 to select the adjacent (to the right, inFIG. 1) transducer 42 through switch matrix 134, thereby directing theoutput of the selected receiving transducer to amplifier 118 via relay116, the state of which the microprocessor selects and controlsaccordingly. The receiving transducer 42 detects the analog mechanicalsignal in the concrete and translates the signal to an analog electricalsignal that is routed by switch matrix 134 to amplifier module 118 viarelay 116. Microprocessor 110 controls amplifier module 118 in this testto amplify, without automatic gain control, the output signal, which isthen delivered to analog-to-digital converter 120, where the signal isconverted to digital form and saved in SRAM 122. The microprocessorcontrols the analog to digital convertor's sampling rate and number ofsamples.

Microprocessor 110 then repeats the process, using the previousreceiving transducer to transmit and using the next adjacent narrowbandtransducer 42 to receive. The microprocessor executes the same sequencewith respect to this new transducer transmitting/receiving pair. Thisprocess repeats for all fourteen transducer pairs in the linear array.

As should be understood, automatic gain control applies a variable gainto incoming signals so that the output amplitude level is within adetecting system's desired signal strength. Automatic gain control isapplied to the outputs of the receiving p-wave transducers 40, thereceiving s-wave transducers 38, and the broadband transducer 180 (FIG.7), which do not carry information as amplitude variation. The outputfrom the receiving p-wave transducers 42 do carry information inamplitude, and microprocessor 110 therefore instructs amplifier module118 to omit automatic gain control from the processing of these signalsand adjust the output voltage to avoid saturation. Because the signalfrom the receiving transducer 42 is acquired without automatic gaincontrol, i.e. with a constant amplification among the receivingtransducers in the array, the data received by the application softwarecorresponding to signals detected by this and subsequent receivingtransducers in the linear array are comparable to each other, so thatthe application software can detect the presence of rebar by looking forpeaks in the amplitude range of the received signals. More specifically,the microprocessor conveys the data it acquires from memory 122 to thecontroller unit, which compares the received signal to a predeterminedthreshold value, and identifies the pressure of rebar if the receivedsignal exceeds the threshold. Alternatively, microprocessor 110retrieves the signal data from memory 122 and analyzes the signal datafor the presence of rebar, reporting to the controller unit the resultsof that analysis.

The threshold level is determined through testing and calibration, andit accounts for concrete's non-homogeneity. As should be understood, thestrength of a signal reflected from rebar will vary inversely to therebar's depth from the concrete surface, but regardless of depth willgenerally result in a peak signal as compared to a signal received fromthe opposing side of the concrete. To calibrate a threshold value, thearray of transducers 42 is made to apply a series of signals to aconcrete sample known to have rebar. This is repeated for samples havingrebar but having different configurations, e.g. different aggregates orcuring history, and the threshold set so that a desired percentage ofthose samples would be identified by application of the threshold. Thisprocess is repeated for samples having rebar at differing depths, forinstance at one foot or two feet increments, and the threshold possiblyadjusted. These thresholds can be programmed at microprocessor 110 orstored in associated memory for its use. Alternatively, rebar depth isassumed only to be at a one foot depth.

In a still further embodiment, the microprocessor collects the outputdata of all fourteen receiving transducers without analysis, and thenupon acquisition of data from all fourteen pairs, uploads the data tothe application software at controller unit 20, which in turn determinesa respective standard deviation of the peak amplitude values of thesignals corresponding to each transducer pair. The application softwarethen determines those transducer pairs having a standard deviation thatis significantly different from most of the other transducer pairs andidentifies those transducer pairs as having identified rebar. The degreeof difference needed to trigger the identification of rebar can bedetermined through testing against concrete samples known to have rebar.Still further, in another embodiment, the application softwaredetermines a standard deviation of the peak amplitude values of thesignals for each individual transducer pair, and identifies anytransducer pair as having identified rebar if the signal for thattransducer pair has a peak value outside a predetermined threshold rangebeyond the standard deviation.

For each transducer pair for which the application software determinesrebar has been identified, regardless of the methodology utilized tomake that determination, the application software or microprocessorstores a marker indicating the identity of the receiving transducer atwhich this occurred.

In a still further embodiment, the control device actuates eachindividual transducer 42 pair through a sequence of excitation pulses ofdifferent amplitude, each therefore resulting in an imparted mechanicalwave of different strength, which in turn are capable of traveling andreturning (in a detectable strength) to different depths. At each testwithin the sequence, the microprocessor uploads the resulting data tothe controller unit and its application software (although it should beunderstood this functionality could remain at the microprocessor), andthe application software applies the threshold value (directly toamplitude or to a standard deviation analysis) in determining thelikelihood of the existence of rebar. If any one of the tests within thesequence for a given transducer pair indicates the likelihood of thepresence of rebar, the application software determines that thistransducer 42 pair has located the likelihood of the presence of rebar.The process repeats for each other operative transducer 42 pair.

At this point, the application software (or microprocessor 110) hasdetermined, for each pair of transmitting/receiving transducers 42,whether the reflection wave detected by the pair's receiving transducerin response to the pair's transmitting transducer is likely to havereflected from a reinforcing bar within the concrete, based on the peaksas analyzed by controller unit 20 or other criteria. As noted above, LEDarray 108 includes a plurality of LEDs that are disposed on housing 14in spatial correspondence to respective transducers 42. That is, theLEDs are arranged in a linear array at a spacing from each other similarto the spacing of the transducer array and in parallel with line 68(FIG. 1D) so that the individual LEDs are generally aligned withrespective transducers 42 in a direction perpendicular to line 68 (FIG.1D). The correspondence between the individual LEDs and their respectivetransducers 42 is stored in the programming and/or memory associatedwith microprocessor 110 and/or the application software. Thus, when theapplication software (or microprocessor) completes the analysis of thetest results from all fourteen transmitter/receiver transducer 42 pairs,the application software sends a signal to the microprocessor, which inturn sends a signal to LED driver 142, via control module 144, causingthe LED driver to actuate those LEDs, if any, corresponding to thetransmitting transducer in those transducer pairs for which themicroprocessor detected rebar. Since this occurs while the user hasdevice 10 pressed up against the concrete surface, the actuated LEDs, ifany, provide the user with a visual indication of a concrete surfaceposition above the actual rebar location. In an alternate embodiment,array 108 is replaced by a single LED, and applicationsoftware/microprocessor 110 actuate this LED whenever any of thetransducer 42 pairs indicates the likelihood of rebar.

As noted above, microprocessor 110 communicates with controller unit 20via a high speed USB controller 138 located on board 100. Upondetermining which transmitting transducers in the linear array returneda signal indicating the likelihood of the presence of rebar, controllerunit 20 outputs corresponding data to microprocessor 110 via USB 138.That is, having determined (a) the time of flight for the mechanicalwave between p-wave transmitting transducer 40 c and first p-wavereceiving transducer 40 a, (b) the time of flight for the mechanicalwave between p-wave transmitting transducer 40 c and second p-wavereceiving transducer 40 b, (c) the time of flight for the mechanicalwave between s-wave transmitting transducer 38 c and first s-wavereceiving transducer 38 a, and (d) the time of flight for the mechanicalwave between s-wave transmitting transducer 38 c and second s-wavereceiving transducer 38 b, and having received from the microprocessor(e) the output from the transmitting/receiving transducer 42 pairs,controller unit 20 (i.e. the application software) calculates p-wavevelocity as the difference between the two p-wave flight times,calculates s-wave velocity as the difference between the two s-waveflight times, calculates compressive strength based on the p-wavevelocity, the s-wave velocity, and density, determines location (interms of transducer location) of any identified rebar, and provides thelocation data to the microprocessor in order for the microprocessor todrive the LED array accordingly.

The application software causes the graphical user interface at screen140 of controller unit 20 in a screen 196 (FIG. 10B) to provideinformation describing the test results (i.e. p-wave velocity, s-wavevelocity, compressive strength, and rebar location) and to provideinstructions to the operator to press device 10 to the concretestructure at a position 90 degrees with respect to the first position,and again press a test button 107. Screen 196 provides the previous testresults at 198, including identifying (by shading) the positions in thelinear array of any transducers 42 whose response data indicated thelikelihood of the presence of rebar in the first test.

The user's activation of a “next” button in the display through a touchscreen causes the application software to repeat the p-wave velocitytest at 160, the s-wave velocity test at 162, the compressive strengthcalculation at 164, the rebar test at 166, and the LED display of rebarposition(s) at 168.

FIG. 10C illustrates a screen display 200 presented by the graphicaluser interface at screen 140, at step 170. The test result datapresented at 202 is the average of the wave velocities and strengthresults (although separate identification of the two test results ismade in another embodiment) determined at steps 148, 150, 152, 160, 162,and 164. At 204, screen 200 displays (by shading) the positions of thosereceiving transducers 42 from which data was received indicatinglikelihood of the presence of rebar in the second test. Screen 140 maybe a touch screen, and upon actuation of a “next” button through thetouch screen, the graphical user interface presents a screen 206 (FIG.10D), instructing the user to initiate the thickness test.

In response, the user inserts the data feed line of impacthammer/broadband transducer (with cables) assembly 176 (FIG. 7) intoLEMO connector jack 104 of device 10 for execution of the thicknesstest. Microprocessor 110 detects the insertion over I/O 114, and themicroprocessor returns a signal to controller unit 20 confirming thatthe device is ready for the test. At 172, the application software sendsa signal back to microprocessor 110 to initiate the test. In response,microprocessor 110 actuates input selector relay 116 to the state atwhich relay 116 directs the output of broadband transducer 180 (i.e. theinput received at jack 104) to the input of amplifier module 118 butdoes not acquire output data from transducer 180 until receiving aninput signal from impact hammer 178 over input/output 114. The graphicaluser interface instructions provided at screen 140 (FIG. 2) may instructthe user to strike the concrete with the hammer's steel ball proximatethe location at which the broadband transducer is coupled to theconcrete surface, i.e. sufficiently close that the resulting mechanicalwave imparted to the concrete by the impact hammer creates a standingwave in the concrete sample detectable by the control device through thebroadband transducer. In one example, the screen display instructs theserver to strike the concrete at a point within fifty mm from theposition of broadband transducer 180. Thus, the user now places theacoustic coupling surface of broadband transducer 180 at the surface ofconcrete sample and, holding the impact hammer, strikes the concretesurface with the steel ball.

The impact hammer produces a mechanical impact on the concrete surface,generating multiple modes of vibration, including p-waves, s-waves, andRayleigh waves. A piezoelectric element is disposed on the steel ballhead that generates an electrical signal when the steel ball strikes theconcrete sample. Impact hammer 178 outputs this signal to microprocessor110, thereby triggering the microprocessor to acquire signal data frombroadband transducer 180 that is directed to memory 122.

As should be understood, the resonant frequency of the standing wavecreated by the hammer impact has a frequency that varies inversely tothe concrete sample's thickness. Shallower samples produce higherfrequencies, whereas thicker samples produce lower frequencies.

As discussed above, the p-wave creates a standing wave between opposingsurfaces of the concrete sample. This wave, and other reflecting waves,excite broadband transducer 180, which converts the mechanical energyinto electrical signals that the broadband transducer outputs toamplifier module 118 via relay 116. Amplifier module 118 amplifies, withautomatic gain control, the analog signal which is then converted todigital form by analog-to-digital converter 120 and stored in memory122.

For a six foot concrete sample thickness, the resonant frequency may bein the range of about 1 kHz. In order to detect such a low frequency,the spectral resolution should be decreased, as shown in FIG. 6. Forexample, if the sampling period of analog to digital converter 120 istwelve microseconds, using a 83.33 kHz clock, and there are 1024 datapoints (N) in the recorded waveform, the duration of the recordedwaveform (N*h) is 12,288 microseconds, which results in a spectralresolution 1/(N*h) of 81 Hz in the signal spectrum. There are 512frequency channels (N/2), and the maximum sample frequency 1/(2*h) is 42kHz. The device can display 1,024 samples in the time domain and up to512 bins (42 k Hz) in the frequency domain. Such a device setup should,therefore, accommodate sample thicknesses at least up to six feet, andit will be understood that accommodation for greater thicknesses can bemade through modification of these parameters.

Microprocessor 110 acquires the output data from memory 122 and forwardsthe data to controller unit 20 via USB device 138, and the softwareapplication analyzes the data to determine the standing wave's resonantfrequency. The software application first sees the data as a time domainwaveform (see the upper portion of FIG. 9) but then executes a FastFourier transform to convert the time domain signal to a frequencydomain signal (as indicated in the lower portion of FIG. 9). Theresonant frequency appears as the highest or first (i.e. lowestfrequency) peak in this waveform. A Hamming window is used because suchmethod produces less ringing in the spectral values, but it should beunderstood that other windowing techniques may be employed.

Accordingly, the software application has determined the concretesample's resonant frequency (f). At 148 and 160, the softwareapplication has determined the p-wave velocities in the concrete sample.Averaging the p-wave velocities (C_(p)), the software application, at174, calculates concrete sample thickness (T) according to the followingformula:

$T = \frac{C_{p}}{2f}$

At this point, the testing sequence is complete, and the applicationsoftware drives the graphical user interface to present a screen 208(FIG. 10E) at display screen 140 that displays the concrete samplethickness determined by the test.

Because of the non-homogeneity of concrete, the tests described hereinmay desirably be performed at various positions on the concrete surface.

The software application may act as an intermediary between the userand/or other computers and the basic computer resources of board 100 andcontroller unit 20, as described, in suitable operating environments.Such software applications include one or both of system and applicationsoftware. System software can include an operating system, which can bestored on controller device 20 and microprocessor 110, that acts tocontrol and allocate resources of these computer systems. Theapplication software takes advantage of the management resources bysystem software through program modules and data stored on either orboth of system memory and other memory sources, for example massstorage.

Moreover, it will be understood from the present disclosure that thefunctions ascribed to controller unit 20 and microprocessor 110 may beembodied by computer-executable instructions of a program, for examplethe application software discussed herein, that runs on one or morecomputers. Generally, program modules include routines, programs,components, data structures, etc. that perform particular tasks and/orimplement particular abstract data types. Moreover, those skilled in theart will appreciate that the systems/methods may be practiced with othercomputer system configurations, including single-processor,multi-processor, or multi-core processor computer systems, as well aspersonal computers and handheld computing devices, microprocessor-basedor programmable consumer or industrial electronic, and the like. Aspectsof these functions may also be practiced in a distributed computingenvironment where tasks are performed by remote processing devices thatare linked through a communications network. However, some aspects ofthe claim subject matter can be practiced on stand-alone computers.

Modifications and variations to the particular embodiments of thepresent invention may be practiced by those of ordinary skill in theart, without departing from the spirit and scope of the presentinvention, which is more particularly set forth in the appended claims.In addition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to belimitative of the invention so further described in such appendedclaims.

What is claimed is:
 1. A system for determining characteristics of aconcrete sample, comprising: a concrete sample; a frame; a plurality oftransducers secured by the frame at predetermined positions with respectto each other and so that coupling surfaces of the transducers aregenerally coplanar with each other; and a control device incommunication with the transducers and configured to actuate a firstplurality of the transducers secured by the frame so that the firstplurality of transducers impart respective mechanical waves in theconcrete sample, and receive output signals from a second plurality ofthe transducers secured by the frame in response to reception of themechanical waves by the second plurality of transducers, wherein thefirst plurality of transducers are arranged in the frame so that an areathat is within a plane parallel to the coupling surfaces and in theconcrete sample and that is bounded within the plane by a collectiveextent of the respective mechanical waves that pass through the planeand that are receivable by the second plurality of transducers has adimension at least as long as a distance between adjacent reinforcingbars in the concrete sample.
 2. The system as in claim 1, wherein thefirst plurality of transducers and the second plurality of transducerscomprise the same transducers.
 3. The system as in claim 2, wherein thecontrol device is configured to receive a respective said output signalfrom each transducer of the second plurality of transducers in responseto reception of a said mechanical wave from a different transducer ofthe first plurality of transducers.
 4. The system as in claim 3, whereinthe control device is configured to actuate the first plurality oftransducers sequentially.
 5. The system as in claim 1, wherein thecoupling surface of each transducer of the plurality of transducerssecured by the frame is circular and has a diameter of about one inch orless.
 6. The system as in claim 1, wherein the transducers of the firstplurality of transducers are aligned linearly with respect to eachother.
 7. The system as in claim 6, wherein the coupling surface of eachtransducer of the plurality of transducers secured by the frame has acenter.
 8. The system as in claim 7, wherein the centers of each pair ofadjacent first transducers are separated by at most about 1.5 inches. 9.The system as in claim 8, wherein the centers of each pair of adjacentfirst transducers are separated by at most about one inch.
 10. Thesystem as in claim 7, wherein the centers of the two first transducershaving the greatest separation are separated by at most about 19.5inches.
 11. The system as in claim 8, wherein the centers of the twofirst transducers having the greatest separation are separated by atmost about 19.5 inches.
 12. A method for determining characteristics ofa concrete sample, comprising: providing a frame, and a plurality oftransducers secured by the frame at predetermined positions with respectto each other and having respective coupling surfaces; placing thecoupling surfaces on a surface of the concrete sample; actuating a firstplurality of the transducers secured by the frame so that the firstplurality of transducers impart respective mechanical waves in theconcrete sample; and receiving output signals from a second plurality ofthe transducers secured by the frame in response to reception of themechanical waves by the second plurality of transducers, wherein thefirst plurality of transducers are arranged in the frame so that an areathat is within a plane offset from the coupling surfaces and in theconcrete sample and that is bounded within the plane by a collectiveextent of the respective mechanical waves that pass through the planeand that are receivable by the second plurality of transducers has adimension at least as long as a distance between adjacent reinforcingbars in the concrete sample.
 13. The method as in claim 12, wherein thefirst plurality of transducers and the second plurality of transducerscomprise the same transducers.
 14. The method as in claim 13, whereinthe receiving step comprises receiving a respective said output signalfrom each transducer of the second plurality of transducers in responseto reception of a said mechanical wave from a different transducer ofthe first plurality of transducers.
 15. The method as in claim 14,wherein the actuating step comprises actuating the first plurality oftransducers sequentially.
 16. The method as in claim 12, wherein thetransducers of the first plurality of transducers are aligned linearlywith respect to each other.
 17. The method as in claim 16, wherein thecoupling surface of each transducer of the plurality of transducerssecured by the frame has a center.
 18. The method as in claim 17,wherein the centers of each pair of adjacent first transducers areseparated by at most a distance equal to about one-half of a minimumseparation between adjacent reinforcing bars in the concrete sample. 19.The method as in claim 18, wherein the centers of each pair of adjacentfirst transducers are separated by at most about 1.5 inches.
 20. Themethod as in claim 17, wherein the centers of the two first transducershaving the greatest separation are separated by at most about 19.5inches.
 21. The method as in claim 19, wherein the centers of the twofirst transducers having the greatest separation are separated by atmost about 19.5 inches.
 22. The method as in claim 12, wherein thedistance corresponds to a maximum separation between adjacentreinforcing bars in the concrete sample.